1. Field of the Invention
The present invention relates to an internal reforming type fuel cell device and a fuel cell generating system whose fuel gas is reformed by effects of reforming catalysts and that electric power is taken out by electrochemical reactions to the outside.
2. Description of the Prior Art
FIG. 1 is a perspective view showing a conventional internal reforming type fuel cell device in partly broken state shown in, for example Laying-Open Publication No. 13576 of 1986 filed in Japan. In FIG. 1, reference numeral 1 denotes a fuel gas electrode. Reference numeral 2 denotes an oxidant gas electrode. The fuel gas electrode 1 and the oxidant gas electrode 2 is arranged in opposing positions to each other through an electrolyte layer 3, and they compose a cell 4. Reference numeral 5 denotes a fuel gas passage arranged in an opposing state to the fuel gas electrode 1. Reference numeral 6 denotes an oxidant gas passage arranged in an opposing state to the oxidant gas electrode 2. Reference numeral 7 denotes a fuel gas side passage-forming material. Reference numeral 8 denotes an oxidant gas side passage-forming material. Reference numeral 9 denotes a separator plate for separating the fuel gas passages 5 and the oxidant gas passages 6 on the occasion of laminating plural cells 4, and for connecting the plural cells 4 in electrically series. Reference numeral 10 denotes a fuel cell stack laminating plural cells 4 through the separator plates 9. Reference numeral 11 denotes reforming catalysts held inside the fuel gas passages 5. In FIG. 1, the fuel gas passages 5 and the oxidant gas passages 6 are arranged in directions intersecting orthogonally with each other (a cross flow system). And, reference numeral 12 denotes a gas manifold for supplying and exhausting fuel gasses and oxidant gasses.
FIG. 2 is a system block diagram showing an outline of peripheral equipments for controlling fuel cells and the temperatures of the fuel cells in a fused carbonate type fuel cell generating system made public in a publication (American GRI report No. FCR-3522-2). In FIG. 2, reference numeral 14 denotes a fuel cell device. Reference numeral 15 denotes an turbine-compressor recovering power from an exhaust gas F exhausted from the fuel cell generating system. The turbine-compressor 15 also raises the pressure of ambient air E and feeds the pressurized air to the fuel cell generating system. Reference numeral 16 denotes a circulating blower circulating a part of oxidant gas D for the temperature control of the fuel cell device 14. Reference numeral 17 denotes a heat exchanger for the temperature control of an oxidant gas side circulating gas circulated by the circulating blower 16. Arrows A and B denote a fuel gas flow fed to and exhausted from the fuel cell device 14 respectively. And, arrows C and D denote oxidant gas flows.
The operation of the internal reforming type fuel cell and the fuel cell generating system shown in FIG. 1 and FIG. 2 respectively will now be described. In the case where steam and fuel such as hydrocarbon are fed in the fuel gas passage 5, the hydrocarbon reacts with the steam by a catalytic reaction with the reforming catalysts 11 and is transformed into hydrogen, carbon monoxide and carbon dioxide gas. In the case where the hydrocarbon is methane, this reaction is represented in the following equation (1): EQU CH4+H2O.fwdarw.CO+3H2 (1)
Produced hydrogen and carbon monoxide pass through holes formed in the fuel-gas-side passage-forming material 7, and is diffused through pores of the porous fuel gas electrode 1. On the other hand, a mixed gas of air and carbon dioxide gas is fed to the oxidant gas passages 6 and it is diffused through pores of the porous oxidant gas electrode 2. Reactant gas whose principal components are said hydrogen and oxygen is consumed by an electrochemical reaction produced among carbonates, electrodes 1 and 2 and the reactant gas itself, where the carbonates are impregnated in the electrolyte layer 3 and are in a melted state near its operating temperature 650.degree. C. Electric potential is established between electric current collectors (not shown in the Figures), and further electric power is taken out to the outside. Now, the reforming reaction produced on the reforming catalysts is an endothermic reaction. The quantity of heat needed to maintain this reaction is fed by the generated heat by said electrochemical reaction.
Generally, it is required for the steady state operation of fuel cells to remove generated heat in cell reactions by cooling. In the internal reforming type fuel cells, both of the cooling using the sensible heat of oxidant gases and the cooling using internal reforming reactions are used together. The temperatures of the fused carbonate type fuel cells are usually controlled near at an average temperature of 650.degree. C. as a result of balancing the decrease of the corrosion amounts of cell construction materials by lowering an operating temperature and the improvements of cell performances by raising the operating temperature ("Handbook of Fuel Cell Performance", DOE Reports Contract of USA, No. EC-77-C-03-1545, May, 1980).
In the operation of the fuel cell devices, it is required to keep the temperatures of the fuel cell devices near said operating temperature by proper temperature control. Namely, thermal energy produced in the operation should be taken away in the steady-state operating condition for keeping the temperatures of the fuel cell devices. On the other hand, the heating of the fuel cell devices constant should be required on the contrary in case of holding on no load or in case of small load to prevent the temperature to decrease.
In case of the fused carbonate type fuel cells, a method circulating a heat medium in a gaseous state is popular as a temperature control method of the fuel cell device 14 for the purpose of the cooling and heating mentioned above. To put it concretely, the temperature of the fuel cell device 14 is controlled by recycling a part of the oxidant gasses through the heat exchanger 17 provided outside of the system. In FIG. 2, a part of the oxidant gas D is recycled to the fuel cell device 14 by the circulating blower 16 and it is utilized as a reaction gas and a cooling gas. The temperature of the fuel cell device 14 is controlled so as to meet the representative temperature of the fuel cell device 14 to a prescribed temperature condition by adjusting the flow rate and the temperature of the oxidant gas C with the operation of the circulation blower 16 and the heat exchanger 17.
A conventional and ordinal temperature control condition is as follows. That is, a large temperature distribution usually exists in a unit cell plane of the fuel cell device 14, for example, temperature range from a minimum value of about 570.degree. C. to a maximum value of about 680.degree. C. exists when the average temperature is about 650.degree. C. Accordingly, the temperature of the fuel cell device 14 having such a large temperature distribution is controlled generally by introducing three reference temperatures, an upper limit temperature, a lower limit temperature and an average temperature, as follows.
1. The upper limit operating temperature is determined by the suppression of the corrosion of the cell construction materials (except for the reforming catalysts).
2. The lower limit operating temperature is determined by the prevention of electrolyte solidification or the improvements of cell characteristics.
3. The average operating temperature is a target average operating temperature as a whole fuel cell device.
The temperature control is executed by adjusting temperature control parameters such as the flow rate and the temperature of the oxidant gas C such that every temperature of the fuel cell device 14 measured by a temperature-measuring means, for example a thermocouple, meets the upper limit operating temperature and the lower limit operating temperature. Further, the temperature is controlled to bring the average operating temperature close to the target average operating temperature after obtaining the average operating temperature of the fuel cell device 14 from the measured temperatures of plural parts. In fused carbonate type fuel cell devices, for example, these temperatures are employed ordinary, namely, 650.degree. C. as a target operating temperature, 680.degree.-700.degree. C. as an upper limit operating temperature, and 500.degree.-540.degree. C. as a lower limit operating temperature.
On the other hand, internal reforming type fuel cell device 14 holds reforming catalysts 11 in a fuel gas passages 5, but the reforming catalysts 11 are poor at heat resistance compared to other cell construction materials. The heat resistance of the reforming catalysts varies on the composition and the kinds of the reforming catalysts, the amounts of attached electrolyte, the composition of the fuel gas, and the like. According to our one embodiment, in case of Ni/MgO catalyst, the activity deterioration of the catalyst became remarkably larger by the operation of more than 5,000 hours in the fuel gas atmosphere containing electrolyte vapor under a temperature condition higher than 650.degree. C. So, it is desirable to set the upper limitation operating temperature of the reforming catalyst to 630.degree. C. or under the same temperature.
In conventional operating methods, one upper limit temperature and one lower limit temperature are set and applied to all operating regions of the fuel cell device 14. Accordingly, newly introducing reforming catalyst upper limit operating temperature (e.g. 630.degree. C.) lower than the conventional upper limit operating temperature to the operating of the fuel cell device 14 requires to operate the fuel cell device 14 in an operating temperature wholly lowered by 50.degree.-70.degree. C. Consequently, the voltage of the stack decreases about 50-100 mV per unit cell average, and generating efficiency also decreases about 3.5-7%. As such large decreases of the cell voltage and the generating efficiency are not acceptable in practice, the concept of the reforming catalyst upper limit operating temperature is not applied actually, and the internal reforming cell is operated near an average operating temperature 650.degree. C.
As mentioned above, conventional operating methods of the reforming type fuel cell devices needs the reforming catalysts kept in higher temperatures than 650.degree. C. Especially, in case of a long operate, the activity deterioration of the reforming catalysts is remarkable. Therefore, the conventional operating methods have the defect that methane in fuel gas flowing in the fuel gas passages in a high temperature operating region becomes being not decomposed by the reforming catalysts and being exhausted from the fuel cell device to the outside of the system after a long operate. This fact was a large factor to lower the efficiency of the internal reforming cell and to determine the lifetime of it.
As the conventional fuel cell devices and the fuel cell generating systems is constructed as mentioned above, the enlargement of the undecomposed methane amounts in exhausted fuel gasses is inevitable with the passage of time due to the deterioration of the reforming catalysts held in the fuel gas passages in a high temperature region in the cell plane. Then, they had defects that the lifetime was short as an internal reforming cell, and the like. Besides, lowering the average operating temperatures of the fuel cell devices for the sake of the protection of the reforming catalysts caused such problems that cell voltages fell much and generating efficiencies fell.